Forcing

What are climate forcings?

The climate of the Earth is affected by a number of factors. These factors are called "forcings" because they drive or "force" the climate system to change. The forcings that were probably the most important during the last millennium were: changes in the output of energy from the sun, volcanic eruptions, and changes in the concentration of greenhouse gases in the atmosphere.

Scientists use various proxies to infer how these forcings have changed over time. The concentration of the isotopes 14C and 10Be, which are preserved in tree rings and ice cores, respectively, depends on solar activity and provide a measurement of this forcing. Volcanic eruptions are preserved as layers of sulfate (SO42-) in ice cores. The concentration of greenhouse gases such as CO2 can be measured in air bubbles trapped in ice cores.

The figure below shows time series of these forcings used in a climate model simulation of the last millennium. The size of these forcings is expressed in terms of Watts (a flux of energy) per square meter of the Earth's surface. Positive forcing warms the Earth, while negative forcing cools the Earth. Proxies do not record these forcings perfectly, so the time series are not known exactly. However, models that use these forcing time series are able to closely match the paleoclimate record of temperature for the last 1000 years. When CO2 is excluded, the models fail to simulate all of the warming of the 20th century, despite the generally successful simulation of the preceding centuries using only natural forcings (i.e., solar and volcanic variations). Model results
(Crowley, T.J 2000)
clearly indicate that greenhouse gas forcing is necessary for explaining the global warming of the 20th century.